The related pages are using NodeIDs (check the CMS_RelationShip table - LeftNodeID, RightNodeID)- this means the data from CMS_Tree table, which is not culture dependent. This means, related pages do not support multilingual setup. They were designed before multilingual support was added. But don't worry, there is a way! You can use the UniSelector form control instead (general UniSelector documentation). And you will configure the selector to work with cms.document page type and set the condition to use current language based on the culture code. Sample screen shot - the WHERE condition is like:
DocumentCulture = '{%EditedObject.DocumentCulture#%}'
And then it could look like in these screen shots:
EN culture
Spanish culture
Hope it helps.